Shion is a kind and gentle person. He has an amazing IQ level, proven by his acceptance into the Special Course and ability to treat and stitch up the bullet wound in Nezumi's arm at age 12. When he was restrained to his own bed by Nezumi, rendered motionless, he seemed fascinated rather than scared of his acquaintance's ability.
Despite his nature, if someone or something he loves is in danger, he doesn't hesitate to save them. He is not easily angered, however when he is he is quick to burst out in violent ways. This is seen when he becomes defensive of Nezumi, attacking and hitting Rikiga when the latter insults Nezumi. He has also shot and killed a guard, who had in turn gravely injured Nezumi. Sion seems to take on a different personality when he does so, in the anime the highlights in his eyes vanish, and he is unaware of what he has done until after the act has been completed. This is proven when he murdered the guard, becoming shocked and frightened once he had pulled the trigger.
This darker nature is only hinted at in the anime and manga, but in the novel series it is addressed more often. Nezumi himself openly admits that he fears Sion, and Inukashi picks up on this as well. In the underground cave, a character named Sasori in the novels calls Sion a "demon".
Though grateful for No. 6, he seems to have a slight doubt about it and the whole system, as noted by Nezumi. He is insecure about many things including his slender body and loves his mother very much. He also seems to like to daydream, but is capable of being aware of what happens in his surroundings when he does.
Nezumi is an outwardly rude, irritable, collected, and upfront person. Inukashi even goes as far as calling him an "outright demon". Nezumi acts very sarcastically, and is a very theatrical individual.
Nezumi has a deep rooted hate for No. 6 and when questioned by Shion, he refuses to give reason in favor of keeping Shion's image of the city. Though introduced as a criminal, it's later revealed that his detainment was for questioning and research. Nezumi is impossibly smart for someone with no formal education; he has the ability to repair and build robots intelligent enough to follow instructions, and has basic math skills on top of impressive reading skills. His knowledge stems from both experience as well as books.
Nezumi is a classic literature buff; his living quarters are filled with books. Nezumi also works at the local theatre, under the stage name "Eve" the only role Nezumi appears performing as is Ophelia from Hamlet, which leads most to the inference that he is cast as predominantly female roles.
Throughout the series Nezumi's personality goes through some drastic changes, his "live for yourself" resolve crumbles which results in him parting ways with Shion at the end of the series, deciding that he had become too attached. [Source: wikia]
You can download the paper craft templates here: No. 6 - Chibi Shion and Nezumi Free Figure Papercrafts Download [Box] [Gavitex]